Crystal oscillator temperature compensating circuit

Abstract

An integratable circuit is utilized for generating a temperature varying control voltage to be applied to a varactor diode to temperature compensate a crystal oscillator. The temperature compensating circuit has a middle range circuit for creating a substantially linear current versus temperature variation in a middle temperature range, a cold-temperature range circuit operative below a predetermined temperature for creating a non-linear current versus temperature variation a hot-temperature range circuit operative above a predetermined temperature for creating a non-linear current versus temperature variation and circuitry for summing said currents and generating a voltage proportional to the current sum. The control circuit, which requires a regulated supply voltage, uses a diode string and differential amplifiers to generate the desired current versus temperature variation.

What is claimed is: 
     
       1. A temperature compensating circuit for an oscillator having a frequency determining crystal, and a voltage variable reactance coupled to the crystal for varying the oscillator frequency in response to a control voltage applied to said reactance, comprising: first current generating means for independently creating a substantially linear current versus temperature variation in a middle temperature range and a non-linear variation in both a hot and a cold temperature range;   second current generating means for creating a substantially non-linear current versus temperature variation in said cold temperature range, said second current generating means including circuitry for independently determining the operative temperature range of said second current generating circuit;   third current generating means for creating a substantially non-linear current versus temperature variations in said hot temperature range, said third current generating means including circuitry for independently determining the operative temperature range of said third current generating circuit;   first current mirror means, coupled to the first current generating means and the third current generating means, for summing currents of said first and third current generating means to produce a first current sum;   second current mirror means, coupled to the first current generating means and the second current generating means, for summing currents of said first and second current generating means to produce a second current sum;   means for summing the currents of said first and second current mirror means to produce a total current sum, and for generating a control voltage with the same temperature variation as said total current sum; and   means for coupling said control voltage to said reactance means whereby said oscillator frequency is maintained at a substantially constant value over all of said temperature ranges.   
     
     
       2. A temperature compensating circuit according to claim 1 wherein said first current generating means includes means for creating an inflection point in said middle temperature range. 
     
     
       3. A temperature compensating circuit according to claim 2 wherein said second and third current generating means includes means for creating a change of slope polarity in said cold and hot temperature ranges. 
     
     
       4. A temperature compensating circuit according to claim 3 wherein said first, second and third current generating means include a temperature sensing element. 
     
     
       5. A temperature compensating circuit according to claim 4 wherein each of said current generating means includes adjusting means for independently determining the magnitude of the control voltage versus temperature variation contributed by it. 
     
     
       6. A temperature compensating circuit according to claim 4 wherein said temperature sensing element is a string of diodes. 
     
     
       7. A temperature compensating circuit according to claim 2 or claim 5 further comprising: means for generating a regulated supply voltage.   
     
     
       8. A temperature compensating circuit according to claim 7, further comprising: adjusting means for determining the temperature at which said inflection point occurs.   
     
     
       9. A temperature compensating circuit according to claim 8 wherein said first, second and third current generating means each include a different amplifier for generating the temperature dependent current. 
     
     
       10. A temperature compensating circuit according to claim 9 wherein all circuitry, excluding all said adjusting means, are integrated on a single semiconductor substrate. 
     
     
       11. A temperature compensating circuit according to claim 9 wherein each said differential amplifier includes a current mirror current source.